This invention relates to optical path-length modulators.
One particularly suitable although not exclusive field of use for which this invention is intended is in a vibration detector of the kind in which a coherent beam of light from, for example a laser, is directed onto an element the vibration of which is to be detected. In such a detector, light which is directed at the object is mixed with light not so reflected in order to derive a Doppler signal denoting the movement of the object. As is further explained in the specification of that patent, it is necessary, in order to distinguish between movement towards and away from the source of light to introduce phase modulation into either the light which is reflected from the object or the reference light which is not so reflected. One way of achieving this modulation is to modulate the path length of the light which is to be subjected to phase modulation. The introduction of modulation of the path length is not a straightforward matter because it is desirable to avoid any undue loss of light and it is also desirable to be able to introduce modulation without affecting any arrangements for focussing the beam of light which is to be projected at the target.